1. Field of the Invention
The present invention relates to microwave radio transmitters and receivers, and particularly to a transceiver which constitutes a complete outdoor radio unit for the Local Multipoint Distribution Service (LMDS).
2. Description of the Related Art
The Local Multipoint Distribution Service (LMDS) is a wireless, broadband technology which is expected to provide a variety of communications services, including video distribution, high speed Internet access, real-time multimedia file transfer, video conferencing, and telecommunications, both for residential and commercial applications. LMDS systems use very high frequencies, the actual spectrum allocation varying from country to country. In the United States, the FCC auctioned licenses for the LMDS in 1998 for two blocks of frequencies located in the 28 GHz band and the 31 GHz band of the spectrum. The short wavelength at such frequencies means that communications in the LMDS is line of sight. Prototype systems use hubs or base stations mounted on towers spaced apart every few kilometers (4-5 kilometers, or about 2.5-3.1 miles apart) to repeat signals and to serve as the centers of cells which service several thousand subscribers, including residences and businesses by transmission to small rooftop antennas. LMDS may be integrated with conventional data transmission techniques, such as twisted pair or fiber optic cables, in a hybrid system. The transmission to subscribers could be by fiber optic cable, but a more likely scenario may be transmission by fiber optic cable to the base unit and retransmission to subscriber units by radio wave, as the advantage offered by LMDS lies partly in the elimination of hardwiring each subscriber unit, and partly in the increased speed of transmission and larger bandwidths by radio wave as opposed to transmission by modem over telephone lines.
Nevertheless, many LMDS systems remain at the prototype level due to technical problems with transceiver equipment and with systems integration. Several problems exist with conventional LMDS transmitter and receiver equipment.
Conventional LMDS transmitter and receiver equipment uses a plurality of discrete modules or components, increasing system costs and entailing loss of efficiency. A conventional transmitter may have a plurality of modules built in separate packages which are connected together. For example, a high power amplifier module may be connected to an upconverter and then attached to a connectorized filter and finally to the antenna feed. This construction approach requires developing techniques for matching the separate modules, and increases costs by the separate packaging of the modules, as well as the loss of transmitter power from coupling the components, with consequent decrease in transmitter power efficiency. Therefore, there is a need for an LMDS transceiver which reduces the number of separate modules in an LMDS transmitter-receiver system.
A further problem with microwave transceiver design is the multiplication of components and internal layout of the transceiver within the housing. Modern microwave transmitter and receiver designs typically use frequency synthesizers having phase lock loop circuitry under the direction of a microprocessor. Such a system is outlined in U.S. Pat. No. 5,844,939, issued Dec. 1, 1998 to Scherer et al. Further, a Product Data Book from Qualcomm Incorporated, CDMA Technologies, of San Diego, Calif. describes the Qualcomm(copyright) CDMA Technologies Phase-Locked Frequency Synthesizer integrated circuit Q3236(copyright) at pages 126-168, and a method of hardwiring the frequency synthesizer to operate at a single frequency without the requirement of a microprocessor at pages 126, 131. Further, connections between circuit boards frequently require a plurality of cables, some being used for high frequency signals and others being used to transfer DC voltages between circuit boards. It would therefore be desirable to reduce the number of discrete components and improve the internal architecture of a transceiver for the LMDS by providing better matching sections.
Another problem encountered in transceivers for LMDS operation is efficient heat transfer. Many transceivers rely upon air convection for the cooling of electronic components, and require a fan to force air flow through the housing. While effective, this design approach increases transceiver size and cost. Further, printed circuit boards in conventional microwave transceivers usually use silver, copper molybdenum, and copper tungsten as carrier material for attachment by conventional solder connections. Such carrier materials are not as efficient heat conductors as copper, and traditional solder connections are not flexible, causing attached components to break during expansion and contraction of the printed circuit boards. Consequently, it is desirable to have a transceiver for LMDS operation with improved heat transfer properties in order to house the transceiver in a smaller package.
None of the above inventions and patents, taken either singularly or in combination, is seen to describe the instant invention as claimed. Thus a complete outdoor radio unit for lmds solving the aforementioned problems is desired.
The complete outdoor radio unit for LMDS is a light weight, compact transceiver featuring planar circuitry. The transceiver uses cross polarization through an orthomodal transducer to provide isolation between the transmitter and receiver. The transceiver includes three printed circuit boards directly attached by thermally conductive epoxy to a heat sink which forms part of the antenna dish/transceiver housing for effective heat transfer. One of the three circuit boards is a millimeter wave circuit board, which is attached to the heat sink by a millimeter ground plane. The millimeter ground plane has a plurality of cavities defined therein which form the lower portion of waveguide assemblies. Two of the waveguides are used as receiver and transmitter transmission lines into the orthomodal transducer, and two of the waveguides are used as bandpass filters. The waveguides interface with the millimeter wave board through microstrips. The millimeter wave board is connected to the other circuit boards by a flex cable using thin film conductive strips to accommodate both high frequency and DC voltages. The transceiver also includes a DCxe2x80x94DC converter in the power supply which is modified to connect the base plate to the heat sink for compactness and improved heat transfer, and a frequency synthesizer with a phase locked loop hardwired to a single reference frequency for compact size. Optionally, the transceiver may be equipped with a five-pole edge coupled bandpass filter constructed from thin film rather than lumped components.
The transceiver includes three printed circuit boards, including a power supply board, an L-band microwave circuit board, and a millimeter wave printed circuit board. The power supply board and the L-band microwave circuit board are directly adhered to a planar ground plane/heat sink which forms part of the antenna housing by a thermally conductive epoxy, and hence are coplanar. The millimeter wave printed circuit board is adhered to a second ground plane by thermally conductive epoxy, the second ground plane being attached directly to the heat sink so that the second ground plane is coplanar with the first two printed circuit boards, giving the transceiver a planar configuration with all three circuit boards attached to the heat sink for effective heat transfer.
Connections between circuit boards are facilitated by a novel flex cable fabricated using thin film techniques and which allows both microwave frequency signals and DC signals to be transferred using the same cable. Optionally, an edge coupled five pole bandpass filter including a dielectric substrate having thin film strips arranged in a precise pattern and a metallic lid placed 50-60 mils from the filter may be provided for rejection of image frequencies and for filtering the local oscillator frequencies from the desired signal in high power transceivers.
The transceiver includes several measures for improved heat sinking and transfer in an essentially planar, air tight housing, including direct attachment of the printed circuit boards to the housing, the use of a gold plated copper heat sink, the elastic attachment of the circuit boards to the heat sink by thermally conductive epoxies, and a novel method of attaching a DCxe2x80x94DC converter to the printed circuit board by extending the mounting pins to the same side as the base plate for direct attachment of the base plate to the heat sink while including a number of via holes in the heat sink for improved heat dissipation.
The transceiver reduces the number of components required in order to achieve a planar circuitry architecture by providing the improved flex cable mentioned above, and by hardwiring the parallel bus on the phase locked loop chip used in the frequency synthesizer in order to eliminate the need for a microprocessor.
Accordingly, it is a principal object of the invention to provide a complete outdoor radio unit for LMDS embodying a transceiver with light weight, compact planar circuitry for improved efficiency and lower cost.
It is another object of the invention to provide an LMDS receiver with antenna launch ports directly on the circuit boards and waveguide filters integrated with the microwave printed circuit boards to eliminate coaxial cable coupling between the transceiver and antenna, thereby reducing component cost and power loss associated with an antenna feed line.
It is a further object of the invention to provide an LMDS transceiver with improved heat sink and heat transfer properties in order to provide a compact transceiver which does not depend upon forced air flow through the housing for cooling of electronic components.
Still another object of the invention is to reduce the number of components in an LMDS transceiver through the use of flex cables which permit connecting intermediate frequency and DC signals between printed circuit boards on the same cable and through hardwiring the phase locked loop in the frequency synthesizer in order to reduce component cost and transceiver size.
Yet another object of the invention is to provide an edge coupled bandpass filter for LMDS frequencies in order to reduce the number of waveguide filters required, thereby reducing transceiver size.
It is an object of the invention to provide improved elements and arrangements thereof for the purposes described which is inexpensive, dependable and fully effective in accomplishing its intended purposes.